Camera with automatic focusing device

ABSTRACT

An automatic focusing device for camera is provided which makes it possible to sequentially photograph a large number of frames while ensuring a high focusing accuracy. A first focusing unit carries out a first focusing operation. After the first focusing unit carries out the first focusing operation, a second focusing unit carries out a second focusing operation. A storage device stores a difference in results obtained by the first and second focusing operations. In sequential shooting, a controller causes the first focusing unit to correct the result of the first focusing operation according to the stored difference, and inhibits the second focusing unit from carrying out the second focusing operation.

CROSS-REFERENCE

This is a continuation of application Ser. No. 10/974,631, filed Oct.27, 2004, now U.S. Pat. No. 7,003,224 which is a continuation ofapplication Ser. No. 10/728,112, filed Dec. 4, 2003, which is now U.S.Pat. No. 6,826,363.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an automatic focusing device for acamera and a method of controlling the same, and more particularly to anautomatic focusing device for an electronic single lens reflex camerawith an interchangeable lens, which is provided with an automaticfocusing means of a phase difference detecting type and an automaticfocusing means of a contrast detecting type, and a method of controllingthe same.

2. Description of the Related Art

Conventionally, as an automatic focusing device for electronic singlelens reflex cameras with an interchangeable lens, an automatic focusingdevice of the phase difference detecting type has been widely used. Theautomatic focusing device of this type carries out focusing of a lensfor focusing a taking lens by a defocus detecting mechanism, which isprovided in the main body of the camera to perform automatic focusingadjustment, as well as motors provided inside the lens or the camera.

Also, an automatic focusing device of a contrast detecting type whichdetects a contrast according to high-frequency components of a signalfrom an image pickup device, and an automatic focusing device which is acombination of the automatic focusing device of the phase differencedetecting type and the automatic focusing device of the contrastdetecting type have been proposed.

The automatic focusing device of the phase difference detecting type iscapable of finding a defocusing direction and a defocusing amount bydetecting a focus only once, and hence only a short period of time isrequired for automatic focusing, and a release time lag is short, whichis an important factor for the electronic single lens reflex camera withan interchangeable lens.

On the other hand, the automatic focusing device of the contrastdetecting type has the advantage that a high focusing accuracy can beensured by detecting a focus according to a signal from the image pickupdevice itself.

However, the automatic focusing device of the phase difference detectingtype is constructed such that a focus detecting sensor is providedindependently of the image pickup device, which may cause a differencebetween a focus detected by the focus detecting sensor and a focusdetected by the image pickup device due to manufacturing errors,environmental change, secular change, etc. of parts includinginterchangeable lenses. To ensure a high focusing accuracy which is ofgreat importance to the electronic single lens reflex camera with aninterchangeable lens, the camera needs to be large-sized and a very highcost is required. The automatic focusing device of the contrastdetecting type has the problem that the release time lag is long since afocus is found by repeating a cycle in which the focus adjusting lensand other parts are moved to detect a focus.

In the case where an automatic focusing device which is a combination ofthe automatic focusing device of the phase difference detecting type andthe automatic focusing device of the contrast detecting type is used, anerror caused by manufacturing errors, environmental change, agingchange, or the like of parts including interchangeable lenses can becorrected after the focus adjusting lens is quickly moved to a point inthe vicinity of a focus, but the number of frames which can be taken persecond in sequential shooting (hereinafter referred to as “the number offrames to be taken in sequential shooting”) cannot be increased sincetwo-stage automatic focusing adjustment is required.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an automaticfocusing device for a camera, which can increase the number of frames tobe taken in sequential shooting while ensuring a high focusing accuracy.

To attain the above object, in a first aspect of the present invention,there is provided an automatic focusing device for a camera, comprisinga first focusing unit that carries out a first focusing operation, asecond focusing unit that carries out a second focusing operation afterthe first focusing unit executes the first focusing operation, a storagedevice that stores a difference between a result of the first focusingoperation carried out by the first focusing unit and a result of thesecond focusing operation carried out by the second focusing unit, and acontroller operable in sequential shooting, for providing control tocause the first focusing unit to correct the result of the firstfocusing operation according to the difference stored in said storagedevice, and to inhibit the second focusing unit from carrying out thesecond focusing operation.

According to the first aspect of the present invention, a differencebetween the result of the first focusing operation carried out by thefirst focusing unit and the result of the second focusing operationcarried out by the second focusing unit is stored, and in sequentialshooting, the result of the first focusing operation carried out by thefirst focusing unit is corrected according to the stored differencewhile the second focusing unit is inhibited from carrying out the secondfocusing operation. As a result, it is possible to increase the numberof frames to be taken in sequential shooting while ensuring a highfocusing accuracy for the second and subsequent frames.

Preferably, the first focusing unit is a phase difference detectingtype, and the second focusing unit is a contrast detecting type.

To attain the above object, in a second aspect of the present invention,there is provided a method of controlling an automatic focusing devicefor a camera, including a first focusing unit that carries out a firstfocusing operation, and a second focusing unit that carries out a secondfocusing operation after the first focusing unit carries out the firstfocusing operation, comprising a storage step of storing a differencebetween a result of the first focusing operation carried out by thefirst focusing unit and a result of the second focusing operationcarried out by the second focusing unit, and a control step of providingcontrol in sequential shooting to cause the first focusing unit tocorrect the result of the first focusing operation according to thedifference stored in the storage step, and to inhibit the secondfocusing unit from carrying out the second focusing operation.

According to the second aspect of the present invention, the sameeffects as in the first aspect can be obtained.

The above and other objects, features, and advantages of the inventionwill become more apparent from the following detailed description takenin conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram schematically showing the construction of anelectronic camera including an automatic focusing device according to anembodiment of the present invention, in which the camera lies in anobservation-through-viewfinder position;

FIG. 2 is a block diagram schematically showing the construction of theelectronic camera in FIG. 1, in which the camera lies in an exposureposition; and

FIG. 3 is a flow chart showing an automatic focusing process carried outby the automatic focusing device of the electronic camera in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described in detail with reference tothe drawings showing preferred embodiments thereof.

FIG. 1 is a block diagram schematically showing the construction of anelectronic camera including an automatic focusing device according to anembodiment of the present invention, in which the camera lies in anobservation-through-viewfinder position.

The electronic camera according to the present embodiment is implementedby a digital still single lens reflex camera with an interchangeablelens which employs a large-sized solid-state image pickup device to pickup an image by primary image formation. The electronic camera accordingto the present embodiment uses a taking lens which is originallyintended for an automatic focusing single lens reflex camera usingsilver halide film.

As shown in FIG. 1, the electronic camera is comprised of a camera mainbody 5, described later, and a taking lens 1 which is removably(exchangeably) mounted on the camera main body 5. FIG. 1 shows a statein which the electronic camera lies in an observation-through-viewfinderposition, and FIG. 2 shows a state in which the electronic camera liesin an exposure position.

The taking lens 1 is comprised of an image pickup optical system 2including a focus adjusting lens, a lens driver 3 which drives the focusadjusting lens to adjust the focus of the image pickup optical system 2,and a lens control circuit 4 which controls the lens driver 3.

The camera main body 5 has a half mirror provided at a central partthereof, that is, it has mainly provided therein a first mirror 6 whichreflects part of object image light sensed by the image pickup opticalsystem 2 and transmits the remaining part of the object image light; aviewfinder screen 7 having a lower surface thereof formed as a diffusingsurface for forming an object image from the object image light guidedby the first mirror 6 and an upper surface thereof formed as a Fresnellens for collecting the formed object image; a pentagonal prism 8 whichreflects the object image collected by the upper surface of theviewfinder screen 7; a viewfinder 9 through which the object imagereflected from the pentagonal prism 8 is observed as a viewfinder image;and a second mirror which reflects the object image transmitted throughthe first mirror 6.

Further, the camera main body 5 has provided therein a focal planeshutter 14 which is disposed behind the second mirror 10, forintercepting a bundle of formed object image light; a solid-state imagepickup device 15 such as a CCD (Charge Coupled Device) or a CMOS(Complementary Metal-Oxide Semiconductor), which is provided behind thefocal plane shutter 14, for forming the object image from the objectimage light from the taking lens 2; and a release switch, not shown. Therelease switch is comprised of a first stroke switch SW1 and a secondstroke switch SW2, and takes a first stroke position corresponding to ahalf stroke (the first stroke switch SW1: ON) and a second strokeposition corresponding to a full stroke (the second stroke switch SW2:ON). The electronic camera carries out a focusing operation when therelease switch lies in the first stroke position, and carries out ashutter releasing operation when the release switch lies in the secondstroke position.

The first mirror 6, second mirror 10, and focal plane shutter 14 arespread out as shown in FIG. 1 when the electronic camera lies in theobservation-through-viewfinder position, and are in a mirror-up state ora retracted state as shown in FIG. 2 when the electronic camera lies inthe exposure position.

Further, as shown in FIG. 1, the camera main body 5 has provided thereinan AF sensor 11 of a phase difference detecting type which detects theobject image transmitted through the first mirror 6 and reflected on thesecond mirror 10, a first focusing unit 12 which calculates a deviationin focus from a signal output from the AF sensor 11, and a cameracontrol circuit 13 that receives the deviation in focus calculated bythe first focusing unit 12 and transmits the same to the lens controlcircuit 4.

The lens control circuit 4 causes the lens driver 3 to drive the focusadjusting lens of the image pickup optical system 2 according to thedeviation in focus calculated by the first focusing unit 12, therebycausing the image pickup optical system 2 to carry out focusing.

A focusing operation is quickly carried out in response to the outputfrom the AF sensor 11 of the phase difference detecting type, that is,according to a defocusing direction and a defocusing amount of the imagepickup optical system 2. The focusing operation, however, contains afocusing error caused by manufacturing errors, environmental change,aging change, etc. of the taking lens 1 and other related parts.

Further, as shown in FIG. 2, the camera main body 5 further has providedtherein an image processing device 16 which performs processing on anoutput signal from the solid-state image pickup device 15, which formsan object image from the object image light from the image pickupoptical system 2, to extract image contrast information, and carries outwhite-balance adjustment, γ-correction, color-matrix adjustment, and soforth to form a photographic image; a second focusing unit 17 whichcalculates a deviation in focus according to the image contrastinformation obtained from the image processing device 16; and a storagedevice 18 which stores a difference in the result of focusing by thefirst focusing unit 12 and the result of focusing by the second focusingunit 17.

The camera control circuit 13 has not only the above described functionbut also a function of receiving the deviation in focus calculated bythe second focusing unit 17 and transmitting the same to the lenscontrol circuit 4. More specifically, the lens control circuit 4 causesthe lens driver 3 to drive the focus adjusting lens of the image pickupoptical system 2 according to the deviation in focus calculated by thesecond focusing unit 17, thereby causing the image pickup optical system2 to carry out focusing.

FIG. 3 is a flow chart showing an automatic focusing process carried outby the automatic focusing device of the electronic camera appearing inFIG. 1.

First, when the release switch lies in the first stroke position withthe first stroke switch SW1 on (“YES” to a step S1), the camera controlcircuit 13 sets a flag 1ST to “1” to indicate that the release switchlies in the first stroke position (step S2), and then executes phasedifference automatic focusing (phase difference AF) (step S3). Morespecifically, in the step S3, the first focusing unit 12 calculates adeviation in focus from an output signal from the phase difference AFsensor 11, and the camera control circuit 13 causes the lens controlcircuit 4 of the taking lens 1 to move the focus adjusting lens of theimage pickup optical system 2 according to the calculated deviation,causing the image pickup optical system 2 to reach a focus position setby the phase difference AF (step S3).

Then, the camera control circuit 13 determines whether the flag 1ST isset to “1” or not (step S4). The camera control circuit 13 thendetermines whether the release switch is kept in the first strokeposition or not (step S5).

If it is determined in the step S5 that the release switch is not in thefirst stroke position (“NO” to the step S5), the step S1 and subsequentsteps are executed again, and on the other hand, if it is determined inthe step S5 that the release switch is kept in the first stroke position(“YES” to the step S5), it is then determined whether or not the releaseswitch lies in the second stroke position with the second stroke switchSW2 on (step S6).

If it is determined in the step S6 that the release switch does not liein the second stroke position, the step S5 and subsequent steps areexecuted again, and on the other hand, if it is determined in the stepS6 that the release switch lies in the second stroke position (“YES” tothe step S6), the camera control circuit 13 carries out a mirror-upoperation for retracting the first mirror 6, the second mirror 10, andthe focal plane shutter 14 to respective shooting positions (shown inFIG. 2) so as to start a shooting sequence (step S7).

Then, the camera control circuit 13 determines whether the flag 1ST isset to “1” or not (step S8). The flag 1ST is set to “1” at first, andhence contrast detecting automatic focusing (TVAF) is executed (stepS9). In the TVAF in the step S9, the image processing device 16 readsout, at a high speed, an image corresponding to a position, at which asignal from the AF sensor 11 is taken out (angle-of-view position), fromthe solid-state image pickup device 15 to obtain image contrastinformation. Then, the second focusing unit 17 calculates a deviation infocus based on the image contrast information, and the camera controlcircuit 13 causes the lens control circuit 4 to move the focus adjustinglens of the image pickup optical system 2 according to the calculateddeviation in focus, so that the image pickup optical system 2 reaches afocus position set by the TVAF.

At a time point the image pickup optical system 2 reaches the focusposition relative to the solid-state image pickup device 15 after thecompletion of the TVAF in the step S9, a difference between the focusposition (the result of focusing) set by the phase difference AF and thefocus position (the result of focusing) set by the TVAF is stored in thestorage device 18 (step S10).

Then, exposure is performed by closing down an aperture, not shown, ofthe taking lens 1 to a set aperture value, driving the solid-state imagepickup device 15 in a normal way, and controlling an exposure time usingan electronic shutter (step S11). On this occasion, the image processingdevice 16 carries out white-balance adjustment, y-correction,color-matrix adjustment, and so forth to form a photographic image.

Further, the focal plane shutter 14 is closed to be returned to thestate of intercepting a bundle of light, the aperture of the taking lens1 is opened up, and the first mirror 6 and the second mirror 10 arereturned to the observation-through-viewfinder position (step S12)

Then, the flag 1ST is reset to “0” (step S13), and the photographicimage formed in the step S11 is stored in e.g. a nonvolatile memory.

It is then determined in a step S14 whether or not the release switch iskept in the first stroke position after the photographic image isrecorded (step S14). If it is determined that the release switch hasreturned from the first stroke position to the original position, theprocess is terminated, and on the other hand, if it is determined thatthe release switch is kept in the first stroke position (“YES” to thestep S14), the step S3 and subsequent steps are executed again to carryout sequential shooting.

If the sequential shooting is carried out in the step S3 and subsequentsteps (“YES” to the step S14), the flag 1ST is reset to “0” (step S13),and therefore, after the phase difference AF is executed (step S3), theprocess proceeds to a step S15 according to the result of thedetermination in the step S4. In the step S15, the image pickup opticalsystem 2 is moved to execute AF correction so as to correct thedifference between the focus position set by the phase difference AF andthe focus position set by the TVAF, which is stored in the step S10.Further, according to the result of the determination in the step S8,the process proceeds to the step S11 to perform exposure, while skippingthe steps S9 and S10 to omit the TVAF.

According to the automatic focusing process described above withreference to FIG. 3, a difference between the focus position set by thephase difference AF and the focus position set by the TVAF is stored,and in sequential shooting, the focus position set by the phasedifference AF is corrected based on the stored difference withoutexecuting the TVAF. Therefore, it is possible to increase the number offrames to be taken in sequential shooting while ensuring a high focusingaccuracy for the second and subsequent frames.

Although in the above described embodiment, the AF correction forsequential shooting is executed after execution of the phase differenceAF, it is more preferable that the taking lens 1 is driven by an amountincluding an AF correction amount when the phase difference AF isexecuted in sequential shooting so that the total period of time forwhich the taking lens 1 is driven can be reduced.

1. A camera comprising: a first focusing unit that carries out a firstfocusing operation based on a phase difference between two images; asecond focusing unit that carries out a second focusing operation basedon contrast of an image; and a shooting unit that carries out oneshooting by causing said second focusing unit to carry out the secondfocusing operation after said first focusing unit carries out the firstfocusing operation and a sequential shooting without causing said secondfocusing unit to carry out the second focusing operation after saidfirst focusing unit carries out the first focusing operation.
 2. Acamera as claimed in claim 1, further comprising: a storage device thatstores a difference between a result of the first focusing operationcarried out by said first focusing unit and a result of the secondfocusing operation carried out by said second focusing unit.
 3. A cameraas claimed in claim 2, wherein said shooting unit carries out thesequential shooting by causing said first focusing unit to correct theresult of the first focusing operation according to the differencestored in said storage device, and inhibiting said second focusing unitfrom carrying out the second focusing operation.